1. Field of the Invention
The present invention relates generally to resistive-particle counting and more particularly to the field of automated particle enumeration and sizing.
2. Description of the Prior Art
Particle-size distribution and concentration are important properties of countless powdered, slurried, or emulsified materials as well as of biological cells, fluid contaminants, and foodstuffs. In processes involving particulate materials, particle size is a critical factor in dynamic process control, equipment evaluation, product quality control, and research and investigation. Present methods of particle-size measurement include microscope counting sieving, adsorption and permeability, and a number of Stokesian methods. Although most of these methods have been automated to varying degrees in recent years with significant improvements in speed and accuracy, there is still a need for instrumentation to reduce frequently inherent tedium, time delay, and error.
One other well-known approach is the optical sensing zone method wherein the passage of a particle through a light beam enables particle numbers to be easily determined, however, the same provides information very difficult to relate in a meaningful way to particle size or shape. By combining the same with continuous-flow techniques, successful techniques have been in clinical use for nearly 10 years, however, disadvantages nevertheless remain. In order to accomplish platelet counting, lysing agents must be used in order to remove the massive interference which would otherwise be caused by erythrocytes. For example, ammonium oxalate, in combination with a detergent, has been previously used, as has 2 M urea to lyse erythrocytes. A small disadvantage with such techniques, however, is that white cells are counted along with the platelets and have to be determined independently for a subsequent correction.
A major alternative to the optical sensing-zone methods is resistive particle counting which has been a valuable analytical technique for the industrial, medical, and scientific communities for approximately 20 years, and which has provided information on particle numbers and particle size with respect to particles ranging in size from approximately 0.4-10 .mu. or more in diameter. The more sophisticated apparatus have in fact been a considerable advance over earlier technology, providing up to seven parameters of clinical interest, mainly with regard to erythrocytes and white cells. In particular, clinical laboratories all over the world carry out enormous numbers of routine measurements of erythrocytes, white cells and blood platelets. Particle volumes are determined too, but this usually requires more time and expensive apparatus and is often not needed for medical diagnostic purposes. The advent of minicomputers has helped automate data reduction and has been very useful for calculating mean particle volumes and giving other statistical information from population histograms.
The major advantage of such apparatus is inherent in the original concepts where electrical information is available for determining both particle numbers and size, and potentially for shape too, with pulse-width analysis. The latter two parameters are of course not presently available with optical measuring systems. Other advantages are that pulse-height analysis is readily performed, and economical minicomputers provide capabilities of extensive data manipulation, aside from exhibiting great speed.
There are, however, several disadvantages of resistive counting. Simple clogging of the sensing orifice has been perhaps the major problem with all counters of such type, and one particular type of apparatus attempts to rectify such a problem through the provision of three orifice tubes each for white cells and erythrocytes, that is, six in all, with a corresponding duplication of circuitry. Comparison of the various data values from each then provides a means of detecting orifice blockage, however, the net result is complex and extremely expensive apparatus, which of course detracts from the inherent simplicity and ease of resistive-counter operation. Other approaches to the problem have also been utilized, such as, for example, a simple backflush plunger which is very valuable for cleaning nearly all blockages, or the specific timing of the orifice flow rate has been another effective way to detect such problems. Still further, special redesign of the aperture tube, as disclosed in U.S. Pat. No. 3,746,976 to Hogg, wherein two orifices are used to minimize eddy currents, and therefore the generated electronic signals, has allegedly resulted in a self-cleaning orifice.
Other problems, however, have included poor resolution and skewed distributions of particles. The latter problem was apparently resolved, for example, with respect to erythrocytes, wherein the provision of longer orifice path lengths gave superior distributions. Several recent approaches to the question of resolution demonstrate that it is often necessary to make complex modifications to the orifices. For example, Thom, as disclosed in U.S. Pat. No. 3,810,010, has designed a clever combination of two orifices, instead of one, wherein the particles are sucked out of the first orifice and surrounded by a sheath of particle-free electrolyte prior to their entry into the second and sensing orifice. The improved resolution stems from the original concept of hydrodynamic focusing, however, Thom's apparatus is quite complex, and is dependent upon skilled glass-blowing and very precise geometries. In addition, due to the suction of the particles into the orifice together with the sheath of particle-free diluent, accurate counts of the detected particles are not obtained.
In U.S. Pat. No. 3,793,587 to Thom and Schulz, the apparatus disclosed therein serves to distinguish leucocytes from erythrocyte agglomerates, and to distinguish between two particles of equal volume but different shape. These goals are achieved by the use of two coaxial orifices of different path lengths, and a feed tube directed to the first orifice. As in U.S. Pat. No. 3,810,010, however, the same requires hydrodynamic sucking, which is a critical feature for success of the invention, and which, as noted hereinabove, draws in a quantity of particle-free diluent around the sucked particles through the first orifice. Consequently, the first orifice does not detect the true concentration of particles in the particle suspension.
Still further, a major disadvantage still remains in that intermittent and manual presentation of samples is still required. That is, individual liquid samples have to be handled and positioned over the sensing orifice prior to each analysis. In addition, relatively-large quantities of blood (1.3 ml) have to aspirate into the machine, mainly to flush away previous samples, and after appropriate dilution, the sample is subject to a static count. On the other hand, flow-counting techniques do have an inherent advantage over the static resistive approach in that they are more flexible in that multiple samples are able to be run in a continuous train towards the sensing zone. In addition, specific manipulations, such as reagent addition, are readily performed on the samples before analysis. Nevertheless, further improvement and/or refinement of such apparatus, and the techniques associated therewith is desired.